1. Field of the Invention
This invention relates to a process for the production of copolymers of acrylonitrile and vinyl chloride in aqueous emulsion.
Copolymers of acrylonitrile and vinyl chloride are suitable for the production of filaments and fibres. In addition to the favourable properties normally encountered, fibres of these copolymers--by virtue of their chlorine content--are substantially non-inflammable and highly flameproof, which makes them eminently suitable for use in the production of wigs, woven imitation furs, children's clothing, carpeting, decorative materials, curtaining and the like.
2. Discussion of Prior Art
It is known that acrylonitrile-vinyl chloride copolymers can be produced by emulsion polymerisation, the polymerisation reaction normally being carried out in the presence of a large excess of vinyl chloride. Chemically uniform copolymers suitable for use as fibre material are only formed by maintaining a defined ratio between the two monomers, acrylonitrile and vinyl chloride, in the monomer mixture throughout the entire polymerisation reaction. The entire quantity of vinyl chloride is normally added at the beginning of polymerisation or is replenished during the reaction. The required monomer ratio is adjusted by controlled addition of the more quickly polymerising acrylonitrile.
In cases where they are used as starting materials for fibres, the polymers have to satisfy stringent requirements in regard to their chemical uniformity. Accordingly, the requisite monomer ratio has to be adjusted very accurately and kept constant during the polymerisation reaction.
The copolymerisation of acrylonitrile and vinyl chloride in aqueous emulsion is initiated by water-soluble redox catalysts, particularly those based on soluble compounds of tetravalent and hexavalent sulphur, the redox system peroxodisulphate/bisulphite being particularly suitable. In addition, it is advantageous to work at relatively low polymerisation temperatures in order to obtain polymers having an adequate molecular weight and satisfactory colour properties.
Various publications (for example Faserforschung und Textiltechnik 14 (1963), page 517; Makromolekulare Chemie 128 (1969) page 83) have shown that the redox system peroxodisulphate/bisulphite requires a low heavy-metal concentration at temperatures of about 25.degree. C. The following reactions take place: EQU S.sub.2 O.sub.8.sup..crclbar..crclbar. +HSO.sub.3.sup..crclbar. .fwdarw.SO.sub.4.sup..crclbar..crclbar. +SO.sub.4.sup..crclbar..+HSO.sub.3. (I) EQU S.sub.2 O.sub.8.sup..crclbar..crclbar. +Fe.sup..sym..sym. .fwdarw.SO.sub.4.sup..crclbar..crclbar. +SO.sub.4.sup..crclbar..+Fe.sup.3.sym. (II) EQU HSO.sub.3.sup..crclbar. +Fe.sup.3.sym. .fwdarw.HSO.sub.3.+Fe.sup..sym..sym.(III)
Reaction (I) between peroxodisulphate and bisulphite, which is accompanied by radical formation, takes place at an extremely low reaction velocity in the complete absence of heavy metal ions at a temperature of 25.degree. C. so that polymerisation cannot be initiated by the starter radicals which result from this reaction. By contrast, in the presence of adequate concentrations of heavy metal ions, generally iron ions, reactions (II) and (III) take place at an adequate velocity, even at low temperatures, with formation of starter radicals, so that technically useful conversions are produced in the polymerisation reaction.
In some cases, the quantities of iron ions which are introduced into the reaction medium by commercially manufactured chemicals are sufficient to produce the required quantity of starter radicals by transferring electrons from the reduction component to the oxidation component (reactions II and III). In other cases, the necessary quantity of iron ions has to be purposefully added to the reaction medium.
Fluctuating concentrations of iron ions give rise to different concentrations of the starter radicals available and hence affect the polymerisation velocity. Accordingly, in the case of monomers having comparatively large copolymerisation parameters, fluctuations in the molecular weight of the copolymers are inevitable. In the course of a continuous or semicontinuous polymerisation process for comonomers showing very different monomer reactivities, as are formed for example in the copolymerisation of acrylonitrile and vinyl chloride, the rates at which the monomers are incorporated also vary. In other words, variations in the chemical composition of the copolymers and an increase in the chemical distribution are inevitable. If, for example during the continuous or semicontinuous copolymerisation of acrylonitrile and vinyl chloride, the rate at which the acrylonitrile is added is kept constant for a modified polymerisation velocity, a shift in the monomer composition occurs during the polymerisation reaction and a copolymer having a varying chemical composition coupled with increased chemical inconsistency is obtained.
In order to be able to obtain constant conversion rates which provide for a constant addition programme for the acrylonitrile in the continuous and semi-continuous polymerisation process, it is essential to adjust a constant, relatively low concentration of iron ions during the polymerisation reaction. However, in cases where the continuous or semi-continuous copolymerisation of acrylonitrile and vinyl chloride is carried out on a commercial scale, fluctuating concentrations of iron ions in the reaction medium will produce fluctuating concentrations of the effective starter radicals.
In the production of copolymers containing more than 80% by weight of copolymerised acrylonitrile, the influence of the concentration of iron ions upon the chemical distribution of the copolymers plays only a secondary role. In cases such as these, the copolymerisation reaction is not based upon the presence of a comonomer in a multiple molar excess relative to acrylonitrile in the polymerisation medium. Instead, comparable concentrations of both monomers or an excess of acrylonitrile is used. If, for example, the reaction of acrylonitrile and vinyl chloride is carried out in such a way that copolymers containing more than 80% of copolymerised acrylonitrile are obtained, the progress of the copolymerisation reaction is controlled by the activity of the acrylonitrile. Although the concentration of heavy metal ions significantly influences the polymerisation velocity, it does not affect the rates at which the individual monomers are incorporated into the polymer chains of the macromolecules and, accordingly, also has no influence upon the chemical consistency of the copolymers obtained.
However, a large molar excess of vinyl chloride is necessary when acrylonitrile and vinyl chloride are reacted to form copolymers containing substantially equal quantities by weight of acrylonitrile and vinyl chloride. In a polymerisation process such as this, the concentration of iron ions influences not only the polymerisation velocity, i.e. the volume-time yield in a reactor, but also the chemical composition and the chemical consistency of the copolymers.
However, heavy metal ions not only act as components of starter systems in the redox activation of the radically initiated copolymerisation reaction, but also affect the quality of the reaction products, even when present in very small quantities. It is known, for example, that iron ions adversely affect the product quality of polymers. In the case of polyvinyl chloride, iron ions, even in small quantities, increase the elimination of hydrogen chloride and reduce both colour and thermal stability (Journal of Polymer Science 12 (1954), 543; Kunststoffe 52 (1962), 398). The harmful effect of iron ions present in the reaction medium during the polymerisation reaction on the product properties of polyacrylonitrile or copolymers containing more than 80% of acrylonitrile is also known and is reflected in a reduced thermal stability and in yellowing of fibres spun therefrom (German Auslegeschrift No. 1,040,242).
In order to eliminate the harmful effect of iron ions upon product quality, processes have been developed for carrying out a polymerisation reaction in the presence of strong complex formers such as, for example, ethylene diamine tetraacetic acid or polyphosphoric acids. In the presence of these complex formers, a distinct improvement in the colour and thermal stability of the polymers was obtained, for example in precipitation polymerisation for the production of polymers containing more than 80% of acrylonitrile.
However, these complex formers influence the polymerisation velocity to a very considerable extent and, when used in a molar excess relative to iron ions, even act as stoppers by excessively reducing the effective concentration of iron ions and hence bringing the polymerisation reaction to a complete standstill. Thus, it is pointed out in U.S. Pat. No. 3,843,749 that, even in low concentrations of 10 ppm, ethylene diamine tetraacetic acid is capable of inhibiting the polymerisation of acrylonitrile with the redox system peroxodisulphate/bisulphite. If, on the other hand, these complex formers are used in a molar deficit relative to iron ions, they are unable to eliminate the harmful effect of the iron, with the result that the polymers are again left with reduced whiteness and diminished thermal stability.
Accordingly, the concentration of these complex formers in the aqueous phase has to be adjusted to a defined, very low value in order both to obtain adequate polymerisation velocities and also to guarantee high thermal stability. However, fluctuations in the polymerisation velocity cannot be avoided with these complex formers unless the concentration of iron ions in the aqueous phase can be kept constant. As already described, these fluctuations in the polymerisation velocity play a secondary role in the production of polymers containing more than 80% of acrylonitrile or in the copolymerisation of monomers showing comparable monomer reactivities, so that polymers of good product quality can be obtained in cases such as these.
These complex formers are not suitable for controlling the copolymerisation of acrylonitrile and vinyl chloride. In this case, fluctuating concentrations of iron ions, such as can occur in industrial polymerisation processes, not only cause fluctuating polymerisation velocities, but also alter the chemical composition and increase the chemical inconsistency of the polymers. Accordingly, even in the presence of these complex formers, fluctuating quantities of iron can make the copolymer unsuitable for use in the fibre sector.